''As a senior member of the group, it is my responsibility to be a role model for younger members,'' she said, ending the four-minute mea culpa with a deep, lingering bow.

The most striking thing about her apology, however, was her appearance. She had shaved her head, a traditional act of contrition in Japan, but perhaps a step too far for a 20-year-old woman whose ''crime'' was to have found herself a boyfriend - 19-year-old Alan Shirahama, a dancer in a boy band.

Minegishi explained she had decided to cut off her long hair immediately after seeing her photograph, her face hidden behind a surgical mask and a baseball cap, in the weekly tabloid Shukan Bunshun on Thursday. That underlined the strict rules to which Japan's young pop stars must adhere to project an image of unimpeachable morals.

In the YouTube video, viewed more than 3 million times, Minegishi said the assignation had been ''thoughtless and immature''. ''Everything I did is entirely my fault,'' she said. ''I am so sorry. I don't believe just doing this means I can be forgiven for what I did, but the first thing I thought was that I don't want to quit AKB48.''

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Minegishi is among the original members of the band, which since it formed in 2005 has built a huge following. Named after the geeky Akihabara district of Tokyo, the group generated more than $190 million in CD and DVD sales.

The AKB48 stable has more than 230 members, with four main bands and several offshoots.

Their act is more saccharine than sexual but the band has courted controversy off stage in the past year.

Last year a TV commercial in which the singers seductively passed confectionery from mouth to mouth attracted complaints and, earlier the same year, the mother of one of them was arrested for allegedly having sex with a minor.

Last month police launched an investigation after a magazine published a photo of a young boy cupping the breasts of former AKB48 member Tomomi Kasai.